Fix up x86_64_sel4 arch after merge

Support a seL4 x86_64 build

fix missing _BSD_SOURCE support in bits/*.h

this is actually rather ugly, and would get even uglier if we ever
want to support further feature test macros. at some point i may
factor the bits headers into separate files for C base, POSIX base,
and nonstandard extensions (the only distinctions that seem to matter
now) and then the logic for which to include can go in the main header
rather than being duplicated for each arch. the downside of this is
that it would result in more files having to be opened during
compilation, so as long as the ugliness does not grow, i'm inclined to
leave it alone for now.

move all limits that don't vary out of bits/limits.h, into main limits.h

protect some limit constants with feature test macros on x86_64

fix the types of some integer constant limits in headers

Port musl to x86-64. One giant commit!